Current standards limit granulocyte storage to 24 hours. Because granu- locyte-colony-stimulating factor (G-CSF) inhibits granulocyte apoptosis, it may be possible to store G-CSF-mobilized granulocytes for longer periods while maintaining cell viability and function. However, G-CSF mobilization increases the yield of granulocytes several-fold and the resulting higher cell concentrations may diminish viability during storage and sig-nificant levels of pyrogenic cytokines maybe produced. Ten granulocyte donors were given dexamethasone (8 mg PO), G-CSF (5 mg/kg SQ), or both and on the next day granulocyte concentrates were collected using a CS3000 blood cell separator. Component cell counts, cell viabilities, pH, and interleukin-1b (IL-1b), IL-6, IL-8, and tumor necrosis factor (TNF) levels were measured over time. Significantly more granulocytes were collected when donors were given G-CSF (4.2 + 2.3 x 10 to the 10th power) or both G-CSF and dexamethasone (6.4 + 2.5 x 10 to the 10th power) compared with dexamethasone alone (2.2 + 1.2 x 10 to the 10th power); p=0.03 and p=0.002, respectively. Storage had little effect on white blood cell (WBC) count. Slight but significant increases in IL-1b and IL-8 occurred after 24 hours and 48 hours. IL-6 and TNF levels did not change. The pH dropped significantly with time in all three types of granulocyte concentrates. Granulocytes mobilized with G-CSF plus dexamethasone were acidic immediately after the collection and pH was below 6.0 after 24 hours. Since acidic conditions are detrimental to cell survival, 13 granulocyte concentrates (six dexamethasone, two G-CSF, and five G-CSF plus dexamethasone) underwent serial twofold dilution in autologous plasma prior to storage. The pH did not remain > 7.0 until the dexamethasone mobilized granulocyte concentrates were diluted eightfold and the G-CSF plus dexamethasone mobilized granulocyte concentrates were diluted 16-fold. To optimize storage pH, mobilized granulocyte concentrates require an eight- to 16- fold dilution, which is operationally impractical. Clinical grade granulocyte preservative solutions are needed to maintain pH during storage and several are being tested.